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\begin{abstract}
Producing referring expressions plays a central role in human communication: people can only communicate if they can agree and establish joint attention regarding the object they are discussing. In a conversational process, speakers and addressees work together in an iterative way to ensure an effective transmission of information \citep{Clark_Wilkes-Gibbs_1986}.  Studies have shown that speakers spontaneously overspecify, meaning that they include more information in their referring expressions than is strictly necessary for their addressee to identify an object. In the literature, there are two competing explanations for this phenomenon: (1) Overspecification is a result of human’s limited cognitive resources, and (2) Overspecification is a useful tool for communication and gives the listener more chances to align with the speaker, compensates for perceptual difficulties, and makes long-term communication more effective. 

% Mangold defines a referential expression as  \emph {``[a phrase] produced by a speaker in order to focus the partner's attention on a certain object within the context of similar ones." } \citep{MangoldPobel88}.

Our hypothesis is coherent with explanation (2) --- we believe that by giving overspecification at the early stages of an exchange, speakers ensure that all properties of the referent are accessible later, even if it takes more immediate resources to process this information. In order to test this hypothesis, we designed an experiment with a situation of second language (L2) acquisition, because it permits us to isolate a situation where no previous alignment between speakers exists.  We created an interactive 3D world in order to teach subjects a set of words in Russian and programmed an instruction-giving system that produced contextualized REs. We compared three groups receiving different amounts of overspecification during the practice phase of the experiment: one receiving only minimal REs, one receiving only overspecified REs, and the third receiving overspecification only if they hesitated when resolving the initial minimal RE. The fact that we performed our experiment using an automated instruction-giving system and a virtual world ensured that all the subjects went through the same learning experience, except for the differences that we explicitly programmed for the three overspecification conditions.

Our results show that the more overspecified referring expressions are, the more the subjects improve their lexical knowledge after the practice of newly acquired lexemes, even if they do not require overspecification directly during the practice session to resolve the referring expressions correctly. From the subjective results we gathered via post-experiment questionnaires, we found that subjects rated the utility of practicing vocabulary using overspecification as being higher than using minimal specification. This confirms our hypothesis that overspecification is a useful tool in long-term communication, and, if used initially, aids comprehension all along the process of communication. Our results are consistent with the repeated findings that show that, for immediate identification purposes, subjects do not rate overspecified REs worse than minimal ones. This cannot be explained by theories that claim that overspecification impairs comprehension and violates listener's expectations \citep{Engelhardt_Bailey_Ferreira_2006,Engelhardt_Bar_11}. Our conclusion is then that the extra time used to process overspecified REs is not a result of violated expectations but of the alignment between speaker and listener, and useful for future communication.

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Keywords: Referring Expressions, Overspecification, Psycholingustic Experiments, Virtual Worlds, Lexical Aquisition.

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